Modern communication devices increasingly use multiple antenna systems, which are often also called smart antenna systems or Multiple Input Multiple Output (MIMO) systems. MIMO communication technique is based on the idea that different replica of one and the same message, which is supposed to be transmitted, are received by different antenna elements. In a subsequent signal processing the corresponding signals from the different antenna elements are then associated with each other in an intelligent way. Thereby, the signal to noise ratio can be increased or a receiver is able to spatially separate different transmitting communication devices from each other.
In the simplified case of for instance intervisibility between a transmitting device and receiving device the separation of radio signals on the basis of their spatial signature can be generally carried out by means of two methods.
a) Beam Forming:
If the radio signals, which are supposed to be distinguished from each other, have the same polarization, beam forming can be used. Thereby, the signals, which are received from at least to different antenna elements, are combined by employing appropriate attenuation factors and/or appropriate phase shifts. This results in an increased sensitivity for receiving radio signals originating from a predetermined direction and a decreased sensitivity for receiving radio signals from another direction. This means that a spatial separation by means of beam forming can be realized by using appropriate weight and phase factors for the signals being picked up by the various antenna elements. By orienting the antenna elements in an appropriate way, radio signals originating from different communication devices and arriving from different azimuth angles can be discriminated from each other.
b) Polarization Separation:
If the radio signals, which are supposed to be distinguished from each other, have a different polarization, these signals can be picked up by means of different antenna elements, which are assigned to the different polarization directions. In case the polarizations are linear polarizations, which are preferably oriented orthogonal to each other, the corresponding radio signals can be distinguished from each other even if their difference with respect to their azimuth angle is very small.
However, in reality there is very often no intervisibility between two communication devices. Therefore, it is often not clear which of the above described methods is most suitable for providing a high quality radio data link between the communications devices. Furthermore, it is not unusual that at least one communication device is moving, such that the characteristics of the radio transmission link frequently varies in time.
There may be a need for providing an antenna arrangement, which allows for a flexible adjustment with regard to different physical conditions of a radio transmission link between different communication devices.